A TEM study of the crystallography and interphase boundary structure of α precipitates in a Zr-2.5 wt% Nb alloy

The crystallography and the interfacial structure of α precipitates (h.c.p.) and β matrix (b.c.c.) boundaries in a Zr-2.5wt% Nb alloy have been studied using TEM. The orientation relationship was found to deviate slightly from the ideal Burgers orientation relationship. The habit plane of precipitates has been determined, and found to be normal to some particular Δgs. The habit plane is characterized by a single set of misfit dislocations, about 10 nm apart, parallel to the long axis of the α plate. The Burgers vector of these dislocations, as determined by g·b contrast analysis, is [0 1 0] with respect to β phase. The experimental observations are discussed in terms of an invariant line strain and the condition of optimal atomic matching in the habit plane as proposed in a recently-developed model. The observations tend to support the predictions of the model.     

Overview no. 63 Non-equilibrium grain boundary segregation of boron in austenitic stainless steel—IV. Precipitation behaviour and distribution of elements at grain boundaries

The distribution of elements and the precipitation behaviour at grain boundaries have been studied in boron containing AISI 316L and “Mo-free AISI 316L” type austenitic stainless steels. A combination of microanalytical techniques was used to study the boundary regions after cooling at 0.29–530°C/s from 800, 1075 or 1250°C. Tetragonal M₂B, M₅B₃ and M₃B₂, all rich in Fe, Cr and Mo, precipitated in the “high B” (40 ppm) AISI 316L steel whereas orthorhombic M₂B, rich in Cr and Fe, was found in the “Mo-free steel” with 23 ppm B. In the “high B steel” a thin (<2 nm), continuous layer, containing B, Cr, Mo and Fe and having a stoichiometry of typically M₉B, formed at boundaries after cooling at intermediate cooling rates. For both types of steels a boundary zone was found, after all heat treatments, with a composition differing significantly from the bulk composition. The differences were most marked after cooling at intermediate cooling rates. In both types of steel boundary depletion of Cr and enrichment of B and C occurred. It was found that non-equilibrium grain boundary segregation of boron can affect the precipitation behaviour by making the boundary composition enter a new phase field. “Non-equilibrium phases” might also form. The synergistic effect of B and Mo on the boundary composition and precipitation behaviour, and the observed indications of C non-equilibrium segregation are discussed.     

Faceting transformation and energy of a Σ3 grain boundary in silver

Grain boundary facets forming at the intersection between a grain boundary and the free surface in diffusion bonded Σ3〈011〉Ag bicrystals during prologed annealing have been characterized crystallographically by metallographic methods. It is shown that the observed faceting has qualitatively the same character as that in Σ3〈011〉 grain boundaries in Cu. The energy of an incoherent Σ3 grain boundary in Ag (210 mJ/m²) is determined from the dihedral angle of the thermal groove and the extrapolated literature data on the surface tension of Ag. The facet geometry is discussed with respect to computer simulation data on the inclination dependence of the energy of Σ3 grain boundaries in Cu. The geometrical stability of a grain boundary near the free surface is considered.     

Characterization of two-phase axisymmetric plume in a gas stirred liquid bath—A water model study

Plume profile and plume cone angle (θ_c) were determined by still photographic technique in a cold model set-up where air was blown into a water bath through an axial nozzle located at the bottom of the vessel. Effects of various operating variables,e.g., gas flow rate(Q), bath height(H), and nozzle diameter(d _n) on the plume profile were investigated. θ_c increased with increasingQ, decreased with increasingH, but was approximately independent ofd. Based on the experimental data the following empirical correlation was developed: whereD is bath diameter, Fr_m is modified Froude number (= 16Q²/π²gd⁴ _nH⋅ _G/(p_L-p_G)), g is acceleration due to gravity, andp _L andp _G are densities of liquid and gas, respectively. G.G. KRISHNA MURTHY, formerly Graduate Student in the Department of Metallurgical Engineering at the Indian Institute of Technology, Kanpur, India.     

Kinetics of grain boundary migration in dilute CuSi bicrystals

Kinetics of grain boundary migration in dilute copper-silicon bicrystals have been investigated by the reversed-capillary technique at temperatures and silicon solute additions ranging from 275–400°C and 50–680 ppm, respectively. Grain boundary displacements increase with increasing time and temperature and are characterized by an apparent activation energy, which increases from 27 to 35 kCal/mole with increasing silicon content. Results are in excellent quantitative agreement with theories of grain boundary migration and indicate that the controlling mechanism is thermally activated transport of silicon across the moving boundary.     

Observations of Surface Relief of Proeutectoid Widmanstätten Cementite Plates in a Hypereutectoid Carbon Steel

Proeutectoid Widmanstätten cementite in a hypereutectoid carbon steel was found to be associated with a surface relief effect. A hot-stage microscope was used for heat treatment and in situ observation. Widmanstätten cementite plates were obtained near the surface of the specimen. The surface relief effect of Widmanstätten cementite plates was quantitatively characterized by atomic force microscopy. It was found that the relief had either a typical tent shape or apex-lost tent shape. The relief tilt angles were of considerable dispersion, ranging from 20 deg to 50 deg.     

Analysis of spatial distribution of grain orientations in deformed 01 ЮT steel

The influence of the rolling parameters on microstructure and texture formation over 01 ЮT steel strip and on its capacity for profound extension is investigated. It is found that the capacity for profound extension in a 01 ЮT steel sample deformed in the austenitic state and cooled in air may be attributed to the higher concentration of small-angle boundaries and special boundaries Σ3 60° <111> than in steel for which rolling ends at subcritical temperatures.     

Effects of the grain boundary segregants P,S, C and N on the grain boundary self-diffusivity in α-iron

Bulk and grain boundary self-diffusivities of Fe were determined by a radiotracer method. The impurities in the starting material were scavenged with titanium, eleven different melts were prepared: pure iron and alloys doped with either 0.012% S, 0.013% C, 0.04% N or 0.01% to 0.56% P. ⁵⁹Fe was electrodeposited on the specimens, diffusion anneals were conducted at temperatures in the range from 550 to 810°C. Depth profiles of the radioactivity were determined by measuring the residual activity after successive abrasions. In the samples doped with P and S the grain boundary concentrations of the dopants were determined by AES. The grain boundary self-diffusion is most rapid in pure iron where the activation energy is (91 ± 4) kJ/mol. All dopants decrease the grain boundary transport, the corresponding activation energies are between 164 and 223 kJ/mol for all doped materials. The effects of the grain boundary segregation on the activation energy of the grain boundary diffusion is discussed.     

A study on precipitation kinetics of sigma phase in a hot-rolled super duplex stainless steel during isothermal aging based on the Johnson–Mehl–Avrami model

Isothermal aging treatment of 2507 super duplex stainless steel (SDSS2507) was conducted at 850°C after solution treatment at 1150°C. The characteristics of sigma (σ) precipitation kinetics in SDSS2507 were discussed and an improved JMA (Johnson–Mehl–Avrami) model had been developed. The results show that, the precipitation mechanism of σ phase in hot-rolled SDSS2507 was quite different to other ordinary DSSs. It can be found that σ phase precipitated via two mechanisms in this experiment. (1) The eutectoid decomposition δ?→?σ?+?γ₂. It dominated the precipitation kinetics of σ phase in aging 0–25?min, and at this time the volume fraction of σ phase increased rapidly. (2) After 25?min, the precipitation of σ phase absolutely relied on the transformation of γ?→?σ which is controlled by the diffusion of Mo and Ni in γ. In this period, the precipitation rate of σ phase was significantly decreased. In this case, the separate modeling on the basis of each transformation mechanism was adopted and then an improved JMA model was developed for the precipitation kinetics of σ phase in the whole aging process. As a result, a good agreement between the experimental data and this developed JMA model can be obtained.     

Analytical modelling of grain growth in metals and alloys in the presence of growing and dissolving precipitates—I. Normal grain growth

The present investigation is concerned with analytical modelling of grain growth in metals and alloys in the presence of growing and dissolving precipitates. In Part I, the different components of the normal grain growth model are outlined and differential equations presented which allow the mean grain size to be calculated as a function of time and temperature. These equations have then been solved for fixed starting conditions by means of analytical and numerical methods to evaluate the matrix grain coarsening behaviour during isothermal heat treatment. The results are presented in the form of novel “mechanism” maps (by utilizing the concept of group variables) which show the conmpetition between the various processes that lead to normal grain growth in metals and alloys in the presence of unstable second phase particles. An analytical approach to the modelling of abnormal grain growth is described in an accompanying paper (Part II).     

TEM study of precipitates in laser clad NiAl bronze

Transmission electron microscopy (TEM) examination of sheet martensite in laser clad NiAl bronze reveals precipitates with diameter of about 20 nm. It is found that the hardness of the sheet martensite shows a pronounced decrease with increasing test temperature. Heat treatment at 673 K for 4 h doubled the size of the precipitates. Convergent beam electron diffraction (CBED) and microdiffraction (MD) in TEM are used to examine the crystal structure of the precipitates in heat treated material. CBED patterns suggest two point groups:4/mmm and m3m. Three MD patterns of the precipitates show complete agreement with B₂ structure. The crystal structure of precipitates in as-clad material is studied by high resolution electron microscopy (HREM) and optical diffraction. The HREM images are a close match with simulated images based on a B₂ crystal structure model.     

Effects of microstructural factors on quasi-static and dynamic deformation behaviors of Ti-6Al-4V alloys with widmanstätten structures

The effects of microstructural factors on the quasi-static tensile and dynamic torsional deformation behaviors in Ti-6Al-4V alloys with Widmanstätten structures were investigated in this study. Dynamic torsional tests were conducted using a torsional Kolsky bar for five Widmanstätten structures, in which microstructural parameters such as colony size and α lamellar spacing were varied by heat treatments, and then the test data were analyzed in relation to microstructures, tensile properties, and fracture mode. Under dynamic torsional loading, maximum shear stress was largely dependent on colony size, whereas shear strain at the maximum shear stress point was on colony size as well as α lamellar spacing. Adiabatic shear bands were found in the deformed area of the fractured torsional specimens, and their width was smallest in the structure whose colony size and α lamellar spacing were both large. The possibility of the adiabatic shear band formation was quantitatively analyzed in relation to microstructural factors. It was the highest in the coarse Widmanstätten structure, which was confirmed by the theoretical critical shear strain (υ _c ) condition for the adiabatic shear band formation.     

A high resolution electron microscope study of a Σ = 9/[110] 141.06° tilt boundary in Au

A structure analysis of a Σ = 9/[110] 141.06° tilt boundary in Au has been performed and found to be in agreement with atomistic computer models. This boundary is naturally occurring in a [110] Au thin film and is formed at the intersection of two microtwins each with a common [110] axis normal to the film plane.     

Effect of heat treatment on the behavior ofδ-ferrite in B410D martensitic stainless steel

The morphology and the evolution ofδ-ferrite existing in B410D slabs,hot-rolled plates,annealed plates and quenched plates were studied through metallographic observation.Results show thatδ-ferrite forms during the solidification process and that it easily grows and increases in quantity during high temperature annealing.Band-shapedδ-ferrite in hotrolled plates is difficult to be eliminated by conventional heat treatment and hard to recrystallize.     

Mechanism of Widmanstätten plate formation in chromium-rich CrNi alloys

The mechanism of formation of the Widmanstätten plates which have recently been found in chromium-rich CrNi alloys, and are quite similar to the typical “Bainitic” plates in copper and silver based alloys, is discussed here by combining experimental data on the composition ofthe Widmanstätten plate with thermodynamic calculations of the stable and metastable phase equilibria. The observed nickel concentration of the Widmanstätten plate is about 30 mass% higher than that of the original b.c.c. (α) matrix, but it is about 4 mass% smaller than that of the f.c.c. (γ) phase in equilibrium with the b.c.c. phase at the start of the plate formation. The possibility of plate-formation mechanisms based on martensitic transformations from b.c.c. or to f.c.c. or from b.c.c. to a metastable phase with the so-called 3R structure is unambiguously excluded from thermodynamic considerations. The precipitation of the metastable phase with the 3R structure is proposed as the most probable mechanism of Widmanstätten plate formation. This mechanism explains the composition of the plate as well as the temperature range and the region of alloy compositions, where the Widmanstätten plate formation has been observed.     

Microstructural evolution in a 17-4 PH stainless steel after aging at 400 °C

The microstructure of 17-4 PH stainless steel at various stages of heat treatment, i.e., after solution heat treatment, tempering at 580 °C, and long-term aging at 400 °C, have been studied by atom probe field ion microscopy (APFIM) and transmission electron microscopy (TEM). The solution-treated specimen consists largely of martensite with a small fraction of δ-ferrite. No precipitates are present in the martensite phase, while spherical fcc-Cu particles are present in the δ-ferrite. After tempering for 4 hours at 580 °C, coherent Cu particles precipitate in the martensite phase. At this stage, the Cr concentration in the martensite phase is still uniform. After 5000 hours aging at 400 °C, the martensite spinodaly decomposes into Fe-rich α and Cr-enriched α′. In addition, fine particles of the G-phase (structure type D8 _a , space group Fm m) enriched in Si, Ni, and Mn have been found in intimate contact with the Cu precipitates. Following spinodal decomposition of the martensite phase, G-phase precipitation occurs after long-term aging.